Well, for around $600, you can get quite a nice setup.
I threw these links together in 5 minutes:
Processor: Intel C2D e6300
RAM: Corsair 2x1GB DDR2 800
Video: XFX 8600
Motherboard: ASUS, Intel socket 775
PSU: Corsair 520watt
Add a few fans ($20 at most), pick an ATX case ($40 to $60), add your drives from the old PC and tada: you have a new, fast PC that supports DirectX 10 and 64-bit computing.
EDIT: If you need a video card to hold yourself over while you save up, just buy a cheap $30 one that you can keep just for testing in the future.